Perform conversions in GetByte

[bgrainger]

MySqlDataReader.GetByte() doesn't perform conversions from any other integral type, but GetInt16, GetInt32, etc. will perform narrowing conversions (with overflow checks) from MEDIUMINT, BIGINT, etc.

SqlClient and MySql.Data don't support implicit conversions in GetByte, but Npgsql and Microsoft.Data.Sqlite do. It doesn't seem like it would be that wrong to be internally consistent with GetInt16/GetInt32.

This may also be helpful for PomeloFoundation/Pomelo.EntityFrameworkCore.MySql#820.

[bgrainger]

Added in 0.57.0.